In recent years, due to multi-functionality of mobile communication terminals, relays, base stations, wireless network devices and satellite communication devices, many studies on improvement in the frequency bandwidth of the transmitters and receivers have been done. RF wideband power amplifiers are desirable as they reduce equipment, power consumption and operating cost for the RF communication infrastructure devices. Thus, a high performance power amplifier able to accommodate a wide range of frequency bands is desired.
RF circuits are an essential element in the terminal and devices receiving and transmitting the radio waves. The RF circuit comprises individual circuits such as Power Amplifier (PA), for handling high frequency signals.
The PA is key device in an RF circuit, which takes the weak high frequency signals from the frequency converter, amplifying the signals to the level of power necessary for the radio system, and supplies the signal to the antenna, e.g. via a duplexer. Improving the bandwidth and implementing multi-band operation in a single PA while maintaining high efficiency characteristics have proven difficult.
One example is within the growing telecommunication industry and services, where a number of co-existing digital mobile telephone standards are currently deployed worldwide, operating in different frequency bands. Therefore, multiband communication systems, that include two or more frequency bands belonging to the two or more communication standards will continue coexisting in parallel and working together to provide coverage and services to the mobile stations.
FIG. 2 illustrates a block diagram of a basic PA module 20 configuration comprising an amplification stage 21 and a matching stage 22.
One problem with a PA is the low output impedance that has to be matched to, e.g. 50Ω. The output impedance of a PA can be described as:
  U  =                    R        ·        I            ⇔      R        =                  U        I            =                        [                      I            =                                          P                                  out                  ,                  watt                                            U                                ]                =                              U            2                                P                          out              ,              watt                                          
For example, the Impedance of the PA operating with 27 dBm out at 2.7 V, is about 14.5Ω which is much lower than 50Ω, which may be the desired impedance in the communication devices.
Matching this low impedance to 50Ω with minimum insertion loss would require two components A and B. See FIG. 1.
The matching of the two components will have low insertion loss since the loss will increase with every added component. The drawback is the high Q of the match. This match will only work well for one frequency, hence the bandwidth will be poor and under no circumstances suitable for broadband PA's.
In order to reduce the Q value and achieve a more broadband match, more components have to be used. Thus, the broadband match will work for a much wider range of frequencies but it requires more components. This may result in a much higher insertion loss and size compared to the high Q matching. The higher insertion loss must be compensated resulting in decreased efficiency, higher current consumption and increased heating. Modern broadband PA's are a trade off when it comes to insertion loss and bandwidth.
A solution for this problem is using different matching depending on the frequency. This may be done either by employing tunable or switchable matching.